Conventional examples of known light-emitting diodes (abbreviation: LED) capable of emitting red, orange, yellow or yellow-green visible light include compound semiconductor LEDs provided with a light-emitting layer composed of aluminum gallium indium phosphide (composition formula (AlXGa1-X)YIn1-YP, wherein 0≦X≦1 and 0<Y≦1). In this type of LED, the light-emitting portion provided with the light-emitting layer composed of (AlXGa1-X)yIn1-YP (wherein 0≦X≦1 and 0<Y≦1) is generally formed on top of a substrate material such as gallium arsenide (GaAs), which is optically opaque relative to the light emitted from the light-emitting layer, and does not have a particularly high mechanical strength.
Recently, in order to obtain visible LEDs of higher brightness and further improve the mechanical strength of the devices, techniques have been disclosed in which the substrate material that is opaque relative to the emitted light is removed from the compound semiconductor layer that includes the light-emitting layer, and then a support layer formed from a transparent material (namely, a transparent substrate) transmitting the emitted light and also having superior mechanical strength to conventional substrates is subsequently bonded to the compound semiconductor layer, and a reflective layer is provided in the vicinity of the bonding interface, thereby producing a junction type LED with improved light extraction efficiency (for example, see Patent Documents 1 to 5).
Furthermore, in those cases where the compound semiconductor layer is bonded to a metal substrate as the support layer, the metal substrate is typically used as the extraction electrode, and an ohmic electrode is formed on the metal substrate side of the compound semiconductor layer, with conduction achieved between this ohmic electrode and the metal substrate.
Conventionally, it has been assumed that the surface of the semiconductor on which the ohmic electrode is formed should preferably be a flat surface in order to ensure favorable bonding strength with the metal substrate.
[Patent Document 1]    Granted Patent Publication of Japanese Patent No. 3,230,638
[Patent Document 2]    Japanese Unexamined Patent Application, First Publication No. Hei 6-302857
[Patent Document 3]    Japanese Unexamined Patent Application, First Publication No. 2002-246640
[Patent Document 4]    Granted Patent Publication of Japanese Patent No. 2,588,849
[Patent Document 5]    Japanese Unexamined Patent Application, First Publication No. 2001-57441